1. Field of the Invention
The present invention generally relates to a dynamic loudspeaker driving apparatus, and more particularly to a dynamic loudspeaker driving apparatus which can reduce levels of distortions in sound from a dynamic loudspeaker.
2. Prior Art
In general, a feedback circuit is arranged between input and output of a power amplifier provided within an amplifier unit of an audio device. By use of this feedback circuit, it is possible to reduce levels of noises and distortion components included in an output signal of the power amplifier.
In addition, the amplifier unit of the audio device may also be provided with a motional feedback circuit (hereinafter, referred to as MFB circuit) which feedbacks a signal corresponding to a vibration of a dynamic loudspeaker so as to reduce a distortion in an operation of the loudspeaker. Theoretically, motional voltage must be applied to a motional impedance of the dynamic loudspeaker, and the MFB circuit negatively feedbacks such motional voltage to the input of the power amplifier.
The above-mentioned motional impedance can be represented by ZM of an electrically equivalent circuit of the dynamic loudspeaker (hereinafter, referred simply to as a loudspeaker) shown in FIG. 1. In FIG. 1, Rv designates a dc resistance component of a voice coil, and Lv designates an inductance component of the voice coil. In FIG. 2, a solid line designates voltage Vi supplied to the dynamic loudspeaker, while a short dashes line designates motional voltage VM which is produced at the equivalent motional impedance ZM representative of a vibration system of the dynamic loudspeaker. The operating distortion of the vibration system of the loudspeaker represents a transient response component of the motional voltage VM.
When the MFB circuit is provided to the dynamic loudspeaker, the negative feedback quantity must become extremely large at the frequencies in the vicinity of a lowest resonance frequency of the dynamic loudspeaker. Hence, it is avoided to provide too much negative feedback quantity for the MFB circuit. In general, a frequency characteristic of the dynamic loudspeaker provided with the MFB circuit has a tendency that the frequency response characteristic must be easily lowered at low frequencies at which the negative feedback quantity must be concentrated. In order to prevent such frequency response characteristic from being lowered at low frequencies, a compensating low-ass filter circuit (i.e., compensating LPF circuit) is conventionally provided at an input side of the dynamic loudspeaker so that the frequency response characteristic will be raised at the low frequencies. However, it is impossible to obtain a perfect compensating characteristic from such LPF circuit.
FIG. 3 shows an example of a conventional dynamic loudspeaker driving apparatus providing the above-mentioned compensating LPF circuit. In FIG. 3, a feedback circuit 2 is connected between input and output sides of a power amplifier 1. In this case, a feedback ratio b of the feedback circuit 2 is set further smaller than one, while a gain of the power amplifier 1 is set further larger than one. Meanwhile, a dynamic speaker 3 and three resistors 4 to 6 constitute a bridge circuit 7. An output signal Es of this bridge circuit 7 diagrammatically corresponds to the motional voltage of the dynamic speaker 3, and such signal Es is detected by a transformer 8. A part of a detection signal outputted from the transformer 8 is feedbacked to the input side of power amplifier 1. In the circuit shown in FIG. 3, the resistors 4 to 6 and the transformer 8 represent the MFB circuit.
In addition, a compensating LPF circuit 9 is provided at input side of the power amplifier 1, and lowering of low frequency characteristics of the circuit shown in FIG. 3 is improved and compensated by the MFB circuit. More specifically, the compensating LPF circuit 9 adequately raises a signal level of input signal Vi in the low frequency range, and the lowering of the low frequency characteristics is improved.
The MFB circuit used in the conventional audio amplifier unit is exclusively used for reducing distortions and noises included in a signal outputted from the power amplifier. However, such MFB circuit is not used for perfectly eliminating distortions due to the transient response of the vibration system of the dynamic loudspeaker at all. In short, the main portion of the conventional dynamic loudspeaker driving apparatus is the negative feedback portion, and the MFB circuit is merely used as an auxiliary circuit of the dynamic loudspeaker driving apparatus.
As shown in FIG. 3, the MFB circuit is a detection circuit constituted by the transformer and the bridge circuit consisting of resistors only. Hence, detection voltage detected by this detection circuit is not identical to the motional voltage in a strict sense. In other words, the detection voltage and the motional voltage are different in waveform, peak value and phase. For this reason, it is naturally impossible to provide much negative feedback, and the over-all frequency characteristics must be irregularly varied. Hence, the characteristics which must be given to the compensating LPF circuit must be extremely complicated, so that it is impossible to compensate the frequency characteristic of the dynamic loudspeaker with accuracy. Therefore, the conventional dynamic loudspeaker driving apparatus can only provide the circuit which can adequately raise the output level in the low frequency range.
As described heretofore, in the conventional audio amplifier unit, it is impossible to perfectly eliminate the all distortions due to the transient response of the vibration system of the dynamic loudspeaker.
Meanwhile, the conventional MFB circuit can use a pressure sensor, a temperature sensor, a microphone or other sensors in order to detect the motional voltage. Instead of using the above-mentioned sensors, a bridge circuit can be used for detecting the motional voltage produced at a voice coil of the loudspeaker, as described before. These techniques are disclosed in a monthly magazine "Radio Technique" published in Japan; October Issue and November Issue in 1984, and February Issue in 1985, for example.
However, in the above-mentioned MFB circuit using the sensors, a phase revolution of a detection output of such sensor must be increased, for example. Hence, there must be a limit of a feedback quantity due to an ability of the sensor. If the feedback quantity is set large, the MFB circuit will oscillate by itself. As a result, the conventional MFB circuit is disadvantageous in that a distortion reducing effect of the loudspeaker must become small.
On the other hand, the MFB circuit using the bridge circuit is disadvantageous in that the circuit constitution thereof must be complicated.
As described heretofore, the conventional dynamic loudspeaker driving apparatus adopting the MFB circuit must detect the motional voltage. For this reason, it is impossible to sufficiently reduce the levels of the distortions of the loudspeaker.